stdenv: Store one package per line in nix-support/propagated-*

This makes those files a bit easier to read. Also, for what it's worth,
it brings us one baby step closer to handling spaces in store paths.

Also, I optimized handling of many transitive deps with read. Probably,
not very beneficial, but nice to enforce the pkg-per-line structure.
Doing so let me find much dubious code and fix it.

Two misc notes:

 - `propagated-user-env-packages` also needed to be adjusted as
   sometimes it is copied to/from the propagated input files.

 - `local fd` should ensure that file descriptors aren't clobbered
   during recursion.

pkgs/build-support/setup-hooks/multiple-outputs.sh

@@ -202,7 +202,7 @@ _multioutPropagateDev() {
 
     mkdir -p "${!propagaterOutput}"/nix-support
     for output in $propagatedBuildOutputs; do
-        echo -n " ${!output}" >> "${!propagaterOutput}"/nix-support/$propagatedBuildInputsFile
+        echo "${!output}" >> "${!propagaterOutput}"/nix-support/$propagatedBuildInputsFile
     done
 }